Cured products of epoxy resin-based compositions are excellent in adhesion, heat resistance and electrical properties and therefore have widespread uses including electric and electronic part materials such as semiconductor encapsulating materials, adhesives and paints. However, they are inherently less flexible and very brittle, so that accumulation of internal stress at curing is inevitable, resulting in problems such as cracks in cast materials encapsulating electronic parts, adhesion failure in adhesives, and cracks and removal in paints. Meanwhile, properties required in the fields of electronic parts and adhesives are increasingly difficult. Properties such as higher strength and reduced elastic modulus are to be achieved without deteriorating the heat resistance.
To solve the above problems, means proposed for reducing the internal stress at curing include (1) a thermosetting resin composition that essentially contains a polyfunctional epoxy compound, a phenolic compound/formaldehyde condensate, and a polybutadiene derivative having at least one carboxyl group, amino group, hydroxyl group, epoxy group, thiol group or isocyanate group per molecule (JP-A-S57-180626); (2) an epoxy resin composition that contains 0.1 to 5% by mass of a synthetic rubber having an epoxy group at a terminal or in the molecule (JP-A-S61-62511); (3) a toughened epoxy resin composition comprising a curable aromatic and/or a curable cycloaliphatic epoxy resin, a curing agent, and an epoxidized low viscosity polydiene polymer having a specific epoxy content wherein the polydiene polymer preferably has a specific structure (JP-A-H09-512039); and (4) an epoxy resin composition comprising the above toughened epoxy resin composition and a hydroxyl-containing substance (JP-A-H10-502696).
The compositions of (1) to (4) have a common technical idea that the flexibility is imparted by internal plasticizing.
The polybutadiene derivative having at least one carboxyl group, amino group, hydroxyl group, epoxy group, thiol group or isocyanate group per molecule, which is disclosed to be substantially useful in JP-A-S57-180626 (1), is a low-molecular weight polybutadiene having a terminal functional group (Examples disclose molecular weights of 2800, 4600 and 4800). JP-A-S61-62511 (2) describes that the synthetic rubber having an epoxy group at a terminal or in the molecule, which is disclosed to be substantially useful, has a low molecular weight particularly preferably in the range of 700 to 3000 (number-average molecular weights disclosed in Examples range from 700 to 3000), and also describes that the epoxy-having synthetic rubber contains much epoxy groups such that the epoxy equivalent is 190 to 220 (the epoxy number is 5.3 to 4.5 meq/g). Such compositions having a low molecular weight and containing a polybutadiene modified with many functional groups are poor in heat resistance. Further, the epoxy-terminated synthetic rubber, which is generally produced by reaction with epichlorohydrin, contains large amounts of impurities such as by-product chloride ions, so that the epoxy resin composition shows lowered humidity resistance and exhibits a corrosive action when used in contact with metal parts.
The epoxy resin compositions of JP-A-H09-512039 (3) and JP-A-H10-502696 (4) are characterized by being imparted with toughness. The epoxidized low viscosity polydiene polymer, which is disclosed to be substantially useful, is a low-molecular weight block polymer containing a vinyl aromatic hydrocarbon (Examples disclose molecular weights of 4960, 5680, 6000 and 6750). The epoxy number in a particularly preferred embodiment is: (i) in the range of 3.5 to 6 meq/g when the polymer contains the vinyl aromatic hydrocarbon at less than 5%, (ii) in the range of 3 to 6 meq/g when the polymer contains the vinyl aromatic hydrocarbon at 5 to 20%, and (iii) in the range of 1.5 to 6 meq/g when the polymer contains the vinyl aromatic hydrocarbon at greater than 20% (the epoxy numbers disclosed in Examples are from 2.2 to 5.2 meq/g). This control of the contents of the vinyl aromatic hydrocarbon and the epoxy group achieves improved compatibility with the epoxy resin. Such compositions containing the epoxidized low viscosity polydiene polymer display flexibility ascribed to the internal plasticizing, but are yet to be improved in maintenance of heat resistance.
It is therefore an object of the invention to provide an epoxy resin composition that is highly heat resistant, has reduced internal stress, and is imparted with sufficient flexibility such that problems such as cracks and separation in adhesive or paint interfaces resulting from internal stress are prevented.